Climate/Ocean dynamics

Changes in the Sea of Okhotsk due to global warming – Weakening pump function to the North Pacific

Kay-Ichiro Ohshima, Takuya Nakanowatari, Takeshi Nakatsuka, Jun Nishioka and Masaaki Wakatsuchi Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan E-mail: [email protected]

Introduction extends downward to 27.4σθ owing to diapycnal mixing caused by strong tidal currents around the It is known that North Pacific Intermediate Water Kuril Straits (Wong et al., 1998). The OSIW (NPIW), characterized by a salinity minimum at outflows to the North Pacific through the Kuril 26.8σθ, is a major water mass at the intermediate Straits and then mixes with East Kamchatka Current level of the North Pacific (e.g., Reid, 1965). Figure 1 Water to form the Oyashio water. The Oyashio water shows the distribution of potential temperature and reaches the confluence of the subtropical and oxygen content on the 27.0σθ isopycnal surface in the subarctic gyres, and then part of the Oyashio water North Pacific. Cold, high oxygen water seems to flows northeastward as the Subarctic Current (SAC), originate from the Sea of Okhotsk. High bounding the subarctic gyre on the south. chlorofluorocarbon concentrations (Warner et al., 1996) also originate from the Sea of Okhotsk. These Warming and Oxygen Decrease Trend distributions suggest that the ventilation source of intermediate water in the North Pacific, including The results of the analyses in this section are based NPIW, is the Sea of Okhotsk. Since large amounts of on Nakanowatari et al. (2007). For the trend sea ice are formed in the Sea of Okhotsk, the densest analyses, we have used all available data for water in the North Pacific (or to be exact, the densest water on the surface of the North Pacific) is produced there. Sinking of this dense water creates an overturning down to the intermediate depths in the North Pacific. The Okhotsk thus plays a role as the pump of the North Pacific.

Figure 2 shows the annual mean cumulative sea ice production calculated from microwave ice information and heat budget (Ohshima et al., 2003). The northwestern shelf is found to be the far highest ice production region in the Sea of Okhotsk. Over the northwestern shelf, a large amount of sea ice is produced due to severe winds from northeastern Eurasia in winter. The sea ice production leads to the formation of cold, oxygen-rich dense shelf water (DSW) with densities of up to 27.0σθ (Shcherbina et al., 2003). The DSW is transported southward into the intermediate layer in the southern Okhotsk Sea, and is mixed with intermediate water coming from the North Pacific. This mixing forms the coldest, freshest and oxygen-richest water in the North Pacific in the density range of 26.8–27.4σθ (Talley, Fig. 1 Horizontal distribution of (a) potential temperature 1991), which is called Okhotsk Sea Mode Water (°C) and (b) dissolved oxygen content (ml/l) on the 27.0σθ (Yasuda, 1997) or Okhotsk Sea Intermediate Water isopycnal surface in the North Pacific. These maps are (OSIW) (Itoh et al., 2003). The signal of OSIW based on World Ocean Atlas 1994 (Levitus et al., 1994).

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United States international joint study of the Sea of Okhotsk from 1998 to 2004, data archived by the Japan Oceanographic Data Center, and profiling float data obtained by the international Argo program from 2000 to 2004. After the quality control, a gridded dataset of potential temperature anomalies on isopycnal surfaces was then prepared for the period 1955–2004, and one of dissolved oxygen for the period 1960–2004.

Figure 3 shows linear trend maps of intermediate water temperature on the 27.0σθ isopycnal surface for the past 50 years. Significant warming trends are observed in the northwestern North Pacific and the Sea of Okhotsk. The warming trend in these regions is most prominent at density 27.0σθ, and the largest warming area exists in the western part (47.5°–55°N, 145°–147.5°E) of the Sea of Okhotsk with an average of 0.68°C/50-yr. The warming trend at 27.0σθ seems to extend along the pathway of the OSIW (see the acceleration potential at 27.0σθ, indicated by green contours in Figure 3). A Fig. 2 Annual mean cumulative sea ice production, significant warming trend is observed in the Oyashio represented by the ice thickness (cm). Estimation is based and SAC regions, but not in the East Kamchatka on the sea ice information from the satellite microwave Current region, i.e., upstream of the Sea of Okhotsk. and heat budget calculation. After Ohshima et al. (2003). Since the intermediate water masses in the Oyashio and SAC regions are largely affected by the OSIW temperature, salinity and dissolved oxygen, taken (Yasuda, 1997), these results indicate that the from the World Ocean Database (WOD01), warming trend in the northwestern North Pacific may observational data obtained by the Japan–Russia– be caused by advection of warmed OSIW.

Fig. 3 Linear trends (colors in °C/50-yr) of potential temperature anomalies at density 27.0σθ (approximately 300–500 m deep) from 1955 to 2004 in the North Pacific. Green contours indicate acceleration potential at 27.0σθ relative to 2000 dbar, derived from our dataset. Modified from Nakanowatari et al. (2007).

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Figure 4 shows the time series of temperature and production in the coastal polynya of the northwestern oxygen content anomalies at 27.0σθ, averaged over shelf. The largest warming trend occurs in the western the Sea of Okhotsk. A positive and negative linear part of the Sea of Okhotsk (Fig. 3), to which DSW is trend is the most significant feature for temperature transported from the northwestern shelf (Fukamachi et and oxygen content, respectively. The temperature al., 2004). Therefore, we suppose that the main cause has increased by 0.62 ± 0.18°C (significant at a 99% of the warming and oxygen-decreasing trends is the confidence interval) during the past 50 years from weakening of DSW production. 1955 through 2004. A similar result is obtained by Itoh (2007). The oxygen has decreased by 0.58 ± Global Warming and the Sea of Okhotsk 0.34 ml/l (significant at a 95% confidence interval) for the past 45 years. The Oyashio and SAC regions DSW formation is caused by sea ice production. also show a warming trend with the magnitude being Thus we examine the change of the sea ice extent or about half of that for the Okhotsk and a decreasing production in the Sea of Okhotsk. Over the last three trend for oxygen with a value less than that for the decades since accurate observation by satellite Okhotsk. The decreasing trend for oxygen in the became possible, sea ice extent in the Sea of Okhotsk Oyashio is consistent with Ono et al. (2001). has decreased by approximately 150,000 km2, corresponding to about 10% of the entire area of the Sea of Okhotsk (blue line in Fig. 5). It has been also revealed that yearly variability of sea ice extent in the Sea of Okhotsk is highly correlated with that of surface air temperatures in the upwind region of the Okhotsk (red line in Fig. 5). Of particular note is that this temperature has risen by approximately 2.0°C over the past 50 years (2.0 ± 1.4°C/50-yr, significant at a 99% confidence level). This value far exceeds the rate of average temperature increase worldwide (0.74°C over the past 100 years), thereby clearly indicating that the region is significantly affected by global warming. The correlation between this temperature and the sea ice extent (r = –0.61, significant at a 95% confidence level) suggests that decreases in the sea ice preceded the beginning of satellite observations.

Fig. 4 Time series of potential temperature (red line) and dissolved oxygen content (blue line) of the intermediate water at 27.0σθ , averaged over the Sea of Okhotsk, during the past 50 years. Closed circles show a 5-yr average with errors at the 95% confidence interval for the averages.

It is shown that warming and oxygen-decreasing trends in the intermediate water are most prominent in the Sea of Okhotsk. Moreover, these trends appear to extend to the northwestern North Pacific along the pathway of the water mass originating from the Sea of Okhotsk. These facts suggest that the trends in the Fig. 5 Time series of sea ice extent anomaly in February (blue line) and surface air temperature anomaly in the northwestern North Pacific are due to preceding upwind region (50°–65°N, 110°–140°E) (red line) for the changes of water-mass properties in the Sea of Sea of Okhotsk. Note that the scale of the sea ice extent is Okhotsk. Intermediate water in the Sea of Okhotsk inverted (the axis on the right). Surface air temperatures retains its cold and oxygen-rich properties by mixing are the mean values between October and March. with dense shelf water (DSW) associated with sea ice Modified from Nakanowatari et al. (2007).

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Although satellite measurements have been available showed that large amounts of dissolved and only since the 1970s, visual observations at the particulate organic carbons are exported from the Hokkaido coast, located on the southern boundary of highly productive northwestern shelf into the sea ice extent in the Sea of Okhotsk, show the intermediate layer in the Sea of Okhotsk through the decreasing trend in the length of the sea ice season outflow of DSW. Moreover, recent observational during the past 100 years (Aota, 1999). These trends data show that in the northwestern North Pacific, of air temperature and sea ice season suggest that sea iron may come from the intermediate water of the ice extent, accordingly sea ice production, has likely Sea of Okhotsk (Nishioka et al., 2007). Iron is an decreased during the past 50 years. During the essential micronutrient for phytoplankton and is thus current global warming, the surface air temperature considered an important factor in determining anomaly in autumn and winter is particularly large biological productivity. It has been recently revealed over northeastern Eurasia (Serreze et al., 2000). The that when dense shelf water sinks to the intermediate DSW production area of the northwestern shelf in the layer in the Sea of Okhotsk, iron is also brought to Sea of Okhotsk is located where the winter monsoon this layer. We hypothesize that this iron is also from northeastern Eurasia directly transports cold air supplied to the western area of the North Pacific and masses. Therefore, intermediate water in the Sea of supports high biological productivity there. If that is Okhotsk, which is ventilated through DSW, may be the case, it is also suggested that if global warming sensitive to the global warming. weakens sea ice production in the Sea of Okhotsk, iron supplies will decrease in the North Pacific as Possible Scenario well as in the Sea of Okhotsk, thus reducing levels of biological productivity and fishery resources. In a nutshell, the Sea of Okhotsk is highly sensitive to global warming: over the past 50 years, the level Figure 6 summarizes our proposal with schematics. of sea ice production has decreased and the amount Because the Sea of Okhotsk is an area sensitive to of dense water sinking has thus declined, thereby the current global warming, it has experienced a weakening the overturning in the North Pacific scale. decrease in the production of sea ice and dense shelf To put it simply, the recent global warming has water in the northwestern shelf during the past 50 weakened the Sea of Okhotsk’s workings as a pump. years. This possibly causes a decrease in the supply of iron in the intermediate layer in the Sea of Recent studies suggest that OSIW has a significant Okhotsk and further in the North Pacific. Finally, this role in material circulation of the intermediate layer might induce the decrease in primary biological in the North Pacific. Hansell et al. (2002) indicated production, fishery resources, and capacity of CO2 that dissolved organic carbons in NPIW originate absorption. from the Sea of Okhotsk. Nakatsuka et al. (2004)

Fig. 6 Schematic showing the impact of the Sea of Okhotsk on the North Pacific through global warming. Bold letters indicate a fact evidenced by observations and analyses. Italic letters indicate a hypothesis. The larger the number of question marks indicates the larger the uncertainty.

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Acknowledgements to ocean interior. J. Geophys. Res. 109: C09S14, doi:10.1029/2003JC001909. This work has been carried out under the cooperation Nishioka, J., Ono, T., Saito, H., Nakatsuka T., Takeda with the Far Eastern Regional Hydrometeorological S., Yoshimura, T., Suzuki, K., Kuma, K., Research Institute, Russia and S. C. Riser of the Nakabayashi, S., Tsumune, D., Mitsudera, H., University of Washington. We would like to thank Johnson, W.K. and Tsuda, A. 2007. Iron supply to the western subarctic Pacific: importance of iron Kazuya Ono and Kyoko Kitagawa for their support export from the Sea of Okhotsk. J. Geophys. Res. in data processing. 112: C10012, doi:10.1029/2006JC004055. Ohshima, K.I., Watanabe, T. and Nihashi, S. 2003. References Surface heat budget of the Sea of Okhotsk during 1987–2001 and the role of sea ice on it. J. Meteorol. Aota, M. 1999. Long-term tendencies of sea ice Soc. Jpn. 81: 653-677. concentration and air temperature in the Okhotsk Sea Ono, T., Midorikawa, T., Watanabe, Y.W., Tadokoro, K. coast of Hokkaido. PICES Sci. Rep. No 12, pp. 1–2. and Saino, T. 2001. Temporal increases of phosphate Fukamachi, Y., Mizuta, G., Ohshima, K.I., Talley, L.D., and apparent oxygen utilization in the surface waters Riser, S.C. and Wakatsuchi, M. 2004. Transport and of western subarctic Pacific from 1968 to 1998. modification processes of dense shelf water revealed Geophys. Res. Lett. 28: 3285–3288. by long-term moorings off Sakhalin in the Sea of Reid, J.L. 1965. Intermediate water of the Pacific Okhotsk. J. Geophys. Res. 109: C09S10, doi:10.1029/ Ocean. Johns Hopkins Oceanogr. Stud. 2, 85 pp. 2003/JC001906. Serreze, M.C., Walsh, J.E., Chapin, F.S., III, Hansell, D.A., Carlson, C.A. and Suzuki, Y. 2002. Osterkamp, T., Dyurgerov, M, Romanovsky, V., Dissolved organic carbon export with North Pacific Oechel, W.C., Morison, J., Zhang, T. and Barry, R.G. Intermediate Water formation. Global Biogeochem. 2000. Observational evidence of recent change in the Cycles 16: 1007, doi:10.1029/2000GB001361. northern high-latitude environment. Climatic Change Itoh, M. 2007. Warming of intermediate water in the 46: 159–207. Sea of Okhotsk since the 1950. J. Oceanogr. 63: Shcherbina, A.Y., Talley, L.D. and Rudnick, D.L. 2003. 637–641. Direct observations of North Pacific ventilation: brine Itoh, M., Ohshima, K.I. and Wakatsuchi, M. 2003. rejection in the Okhotsk Sea. Science 302: 1952–1955. Distribution and formation of Okhotsk Sea Talley, L.D. 1991. An Okhotsk Sea water anomaly: Intermediate Water: an analysis of isopycnal implications for ventilation in the North Pacific. climatology data. J. Geophys. Res. 108: 3258, Deep-Sea Res. Part A 38(suppl.): S171–S190. doi:10.1029/2002JC001590. Warner, M.J., Bullister, J.L., Wisegraver, D.P., Levitus, S., Burget, R. and Boyer, T. 1994. World Gammon, R.H. and Weiss, R.F. 1996. Basin-wide Ocean Atlas 1994. NOAA Atlas NESDIS 3 and 4, distributions of chlorofluorocarbons CFC-11 and U.S. Department of Commerce, Washington, DC. CFC-12 in the North Pacific. J. Geophys. Res. 103: Nakanowatari, T., Ohshima, K.I. and Wakatsuchi, M. 2849–2865. 2007. Warming and oxygen decrease of intermediate Wong, C.S., Matear, R.J., Freeland, H.J., Whitney, F.A. water in the northwestern North Pacific, originating and Bychkov, A.S. 1998. WOCE line P1W in the Sea from the Sea of Okhotsk, 1955–2004. Geophys. Res. of Okhotsk: 2. CFCs and the formation rate of Lett. 34: L04602, doi:10.1029/2006GL028243. intermediate water. J. Geophys. Res. 103: 15625– Nakatsuka, T., Toda, T., Kawamura, K. and Wakatsuchi, 15642. M. 2004. Dissolved and particulate organic carbon in Yasuda, I. 1997. The origin of the North Pacific the Sea of Okhotsk: Transport from continental shelf Intermediate Water. J. Geophys. Res. 102: 893–909.

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